International patent publication WO 99/52004 discloses catadioptric optic projection objectives which include a plurality of aspheric lens surfaces. For example, the projection objective shown in FIG. 4 includes 12 aspheric lens surfaces for 15 lenses. The manufacturing costs of aspheric lens surfaces with the accuracy required in microlithography are very high. Accordingly, these objectives are of little interest in the marketplace because of the many required aspheric lens surfaces.
European patent publication 0 322 201 discloses an optical projection system especially for photolithography. The projection objective known from this publication includes five lens groups. The first, second, third and fifth lens groups each have only one lens. In part, the lenses are provided with aspheric lens surfaces. An aspheric object end mounted lens surface of the fifth lens group follows an aspheric lens surface mounted in the fourth lens group at the image end.
European patent publication 0 851 304 discloses the adjacent mounting of aspheric lens surfaces in a projection objective. These aspheric lenses are supported so as to be displaceable in the radial direction. The projection objective is matched via the relative movement of the lenses. The aspheric lens surfaces are especially rotationally unsymmetrical because of the possibility of displacing the aspheres in radial direction with respect to each other. Because of the movable support of the aspheric lenses, this arrangement is not suitable for every projection objective because projection objectives designed especially for short wavelengths react sensitively to the smallest position change of the individual lenses. Accordingly, the position stability, which is achievable because of the special support of the lenses, is not sufficient in order to reliably ensure a good imaging quality.
German patent publication 198 18 444 discloses a projection optic arrangement having a purely refractive projection objective which includes six lens groups G1 to G6. In this projection objective, the lens groups G1, G3 and G5 have positive refractive power. The lens groups G2 and G4 have negative refractive power. To correct imaging errors, some lenses, especially in the fourth and fifth lens groups, have aspheric lens surfaces.
German patent publication 199 42 281.8 discloses additional projection exposure objectives which have six lens groups. The second lens group and the fourth lens group have negative refractive power. In the projection objectives known from this publication, lenses having aspheric lens surfaces are preferably arranged in the first three lens groups. A minimum number of spherical lens surfaces are arranged between the aspheric lens surfaces. This minimum spacing between the aspheric lens surfaces appears necessary so that the utilized aspheric lenses can develop their optimal effect.
From U.S. Pat. No. 4,871,237 it is already known to match an objective in dependence upon barometric pressure via the refractive index of a fill gas in the lens intermediate spaces. For example, spherical aberration, coma and other imaging errors can be corrected with a suitable combination of intermediate spaces.
U.S. Pat. No. 5,559,584 discloses introducing a protective gas into the intermediate spaces between a wafer and/or a reticle and the projection objective in a projection exposure system for manufacturing microstructured components.
It is an object of the invention to provide a projection objective and a projection exposure system as well as a method for manufacturing microstructured components. These components are improved with respect to the imaging quality and the resolution capacity. Furthermore, it is an object of the invention to reduce manufacturing costs.
The projection objective of the invention defines a maximum lens diameter (D2) and includes: a plurality of lenses defining an object plane (0) and an image plane (0xe2x80x2); at least two of the lenses having respective mutually adjacent lens surfaces which are aspheric to define a double asphere; the double asphere being mounted at a distance from the image plane (0xe2x80x2) corresponding at least to the maximum lens diameter (D2); the lenses of the double asphere defining a mean lens diameter; and, the mutually adjacent lens surfaces being mounted at a spacing from each other which is less than half of the mean lens diameter.
In a projection objective having a plurality of lenses, the measure of arranging the double asphere at a spacing of at least the maximum lens diameter of the objective away from the image plane (especially the wafer plane), improves the imaging qualities of a projection objective in comparison to a projection objective without such double aspheres. In the above, at least two mutually adjacent mounted lens surfaces are aspheric and this is identified as a double asphere. The spacing between the aspheric lens surfaces of the double asphere is maximally half the lens diameter of the mean diameter of the double asphere. The numerical aperture can especially be increased in a refractive projection objective with the use of at least one double asphere in that the first convex form is shortened so that, at a constant length of the projection objective, the third convex form experiences an increase of the numerical aperture of approximately 0.03 to 0.05.
Especially in purely refractive projection objectives, the use of double aspheres with an arrangement in the first three lens groups has been shown to be especially advantageous.
In lithographic objectives, there are particular locations, which operate especially well on difficult to control aberrations, when these locations are aspherized. Precisely here it is purposeful to utilize especially the effectiveness at the corresponding location via a complex aspheric function. The region of the first restriction and the end of the second convex form as well as regions behind the diaphragm are predestined. Since the technical realization of complex aspheres is subjected to technical limits, the complex asphere functions are realized by means of double aspheres. In this way, a still more extensive correction is possible and the aspheres of the double asphere are technically realizable.
Furthermore, it has been shown to be advantageous to provide aspheric lens surfaces as aspheric lens surfaces of the double asphere. The radius of the aspheric lens surfaces of the best-fitting spherical lens surface (identified as the profile radius) differ very little. Preferably, the reciprocal values of the profile radius or radii of the double aspheres deviate less than 30% from each other. As a reference value, the reciprocal value of the larger radius in magnitude is applied.
It has been shown to be especially advantageous that the apex radii of the aspherical lens surfaces of the double aspheres differ by less than 30% with reference to the larger apex radius in magnitude.
In the area of microlithography, the developmental work is directed to increasing the resolution. On the one hand, the resolution can be increased by increasing the numerical aperture, utilizing ever smaller wavelengths and even by correcting the occurring imaging errors. For an increase of the image end numerical aperture, the last convex form of the objective arranged at the image end is increased. However, it is problematic that only a fixed pregiven space can be made available for the objective. Accordingly, in order to provide a larger numerical aperture, it is therefore necessary to save space in other regions of the objective.
It has been shown to be advantageous to provide the space needed for increasing the numerical aperture by shortening the first convex form. With the first convex form, especially the input telecentrics and the distortion are corrected. By utilizing double aspheres, it is possible to correct the input telecentrics as well as the distortion with ease and at a short distance. With the double asphere, a variable adjustment of the location is made available at a short distance. With the possibility of varying the location, the distortion can be corrected. Especially the input telecentrics is corrected because the angle can be flexibly influenced.
Corrective means has already been made available in the input region of the objective especially with the use of a double asphere in a refractive projection objective in the region of the first two lens groups, that is, up to and including the first lens group of negative refractive power. Accordingly, the corrective means, which is required in the third convex form, are reduced for ensuring a uniform or constant imaging quality.
Furthermore, by providing a double asphere in the forward region of the objective, especially up to the second restriction, the number of lenses is reduced. This operates advantageously on the manufacturing costs.
In purely refractive projection objectives, it has been shown to be advantageous to provide aspheric lens surfaces in the forward region of the objective ahead of the second restriction to improve the imaging quality. For example, for a numerical aperture of 0.83, the deviation from the wavefront of a spherical wave is reduced to less than 6 mxcex with a field of 8xc3x9726 mm2 referred to 248 nm.
The imaging characteristics of the objective can be changed because of fluctuations of the atmospheric pressure. In order to compensate for such pressure fluctuations, it has been shown to be advantageous to charge an intermediate space between two lens surfaces with pressure in a targeted manner so that pressure changes, especially of the atmospheric pressure, can be compensated. Furthermore, the targeted application of pressure can be used for a further reduction of imaging errors.
Furthermore, it has been shown to be advantageous to provide at least one of the end plates with a pressure manipulator so that a curvature of the plate or lens can be generated with a two-sided application of pressure of the particular lens or the particular plate. For a three-point support of the end plate and an application of pressure of the gas space, the three-waviness during operation is corrected in a targeted manner by means of the through-bending of the end plate. With an n-point support, an n-waviness can be corrected.
A force, which is directed in the z-direction, for curving the lens can be introduced via coaxially mounted actuators, especially, piezos. The force, which is introduced by the actuators, is directed to the lens center point.